For virtual try-on, one photo is enough, as long as it is the right one: full body, straight on, even light, plain background, fitted clothing.
For generating a set of photos of yourself, more reference images help, but variety matters far more than volume. A handful of clear, recent, well-lit selfies across a few different angles and expressions will outperform twenty near-identical shots taken in the same room on the same day, because the model needs to learn what your face does rather than what one pose looks like.
Quality rules are consistent regardless of count. Use recent photos so the output looks like you now. Avoid heavy filters and beauty-mode smoothing, which have already altered your features. Avoid sunglasses, hats pulled low, and anything obscuring your face. And include at least a couple where you are not doing the same expression, since a set of identical smiles produces a set of identical results.
One habit worth adopting: shoot your reference photos deliberately, once, in good light, rather than scraping whatever is newest in your camera roll. It takes a few minutes and improves every generation afterwards.